Electronic motor control system



April 20, 1948. F. o. WISMAN 2,440,132

ELECTRONIC ROTOR CONTROL SYSTEI Filed April 12, 1944 iatcnted Apr. 20, 1948 ELECTRONIC MOTOR CONTROL SYSTEM Franklin 0. Wisman, South Bend, Ind., assignor to Bendix Aviation Corporation, South Bend, Ind., a corporation of Delaware Application April 12, 1944, Serial No. 530,714

2 Claims.

This invention relates generally to a control system but more particularly to temperature control by electrical means.

In heat treating furnaces or in various types of ovens it is desirable to have a temperature control mechanism that is not only simple to operate and maintain but a control that is responsive to exceedingly minute temperature variations. It is also advantageous to have an electrical temperature control device which does not contain batteries which must be replaced from time to time, nor contactors which are a source of trouble and require constant attention.

It is therefore an object of this invention to provide an electrical temperature control which when once calibrated for a particular temperature range requires a minimum of attention.

A further object of this invention is to provide a temperature control that is simple to adjust for the various temperature settings desired.

Another important object of the invention resides in the provision of an electrical temperature control system which is dependent on temperature change of resistivity of certain materials for its operation.

A still further object of the invention resides in the provision of an electrical temperature control system which requires a minimum of moving parts and which can be manufactured at a relatively low cost.

Another highly important object of the invention is to provide a temperature control incorporating. all of the merits hereinbefore mentioned and which will respond proportionately to temperature deviation from the predetermined value.

(Ether objects and advantages of the inveniion will become evident from the specification and drawing, in which:

Figure l is a diagrammatical view of the temperature control system of this invention; and

Figure 2 shows one form of clutch suitable for use in the system of Figure l.

The temperature control system of this invention is supplied by an alternating current source which is connected to a heater-cathode type tube l0, which combines in one bulb a halfwave rectifier and a beam power amplifier. A heater-cathode type pentode tube 12, which provides the first stage of amplification for the system, is connected to the half-wave rectifier of tube Hi. The beam power amplifier of tube III, which provides the second stage of amplification, is connected to the half-wave rectifier of the same tube through external connections. A grid bias resistor H is connected into the plate return circuit of the beam power amplifier of tube H1. The resistor I4 is shunted with a suitable by-pass condenser 16, having a capacitance such that the reactance of the condenser at the frequency of the si nal is much lower than the resistance which it is shunting. A grid bias resistor 18, shunted by a suitable condenser 20, is connected to the plate return circuit of the amplifier tube H.

A condenser 24 is connected between the screen grid and the plate return of tube 12 to reduce the grid-to-plate capacitance of the tube and increase the effectiveness of the screen grid. A resistance 26 in series with the screen grid maintains the potential of the screen at a value commensurate with the plate potential. A load resistance 28 in the anode lead of tube l2 couples the two amplifiers so that; any variation of the anode current in the resistance 28 produces across it a varying voltage drop which is impressed on the grid of tube ID. A blocking condenser 30 is interposed between the plate of tube l 2 and the grid of tube ID to preclude the high plate voltage of the former tube from being applied to the grid of the latter tube. A resistor 34 provides a grid leak for tube Ill. A filter condenser 36 is connected to the cathode of the rectifier for smoothing out the pulsating direct current.

A two-phase motor 38, having a winding 40 supplied by the output of the two stages of amplification, and a winding 42 excited from the alternating current source, drives a control arm 44 through a speed reducer 46 comprising two gears 48 and 50. The control 44 is secured to a shaft 52 which is drivably connected to the gear through a slip clutch arrangement 54 which allows for continued rotation of gear 50 when arm 44 engages either of stops 56. The clutch arrangement, as best shown in Figure 2, comprises the shaft 52 having secured thereto a flange portion 58 which is urged into frictional engagement with the gearill by a spring 60. This provides a frictional drive between gear 50 and shaft 52. The design of the motor is such that it will not operate on single phase current.

' An input circuit for the first stage of amplification comprises in general a bridge including a plurality of resistance legs. A thermistor 62. adapted to respond to ambient temperatures, forms one leg of said bridge and a fixed resistance 64 forms another leg. A third leg 01' the bridge includes a,variable resistance 66 and a fixed resistance 68, which is connected through the thermistor 62 to the variable resistances l0 and 12 which form a fourth leg of the bridge. The bridge is balanced at a predetermined temperature condition of the thermistor so that for any temperature deviation the thermistor rheostat or resistance 12 of the bridge an amount proportional to the temperature deviation encountered by the thermistor, so as to restore the electrical balance of the bridge, when the motor 38 drives the control arm 44, which is secured to shaft 52. A winding 14 is disposed in the two-phase motor with respect to winding 42 so as to receive an induced voltage from the motor winding. The transformer action oi these two windings provides an alternating voltage supply or signal for the bridge which signal is impressed on the grid of tube l2 when the bridge becomes unbalanced. To-obtain an out of phase relationship with respect to the current supplied to the windings 40 and 42 of motor 38, a condenser 16 is interposed in the circuit connecting the winding 14 to the bridge. This phase displacement or phase shift is essential for the motor operation and may be provided by other methods familiar to those skilled in the art.

Operation of the temperature control is as follows:

With the variable resistances of the bridge adjusted to give a balanced condition at a predetermined temperature, assume that the temperature to which the thermistor S2 is subjected when the balance is had has changed or deviated. This will cause the resistance of the thermistor to change which will create an unbalance of the bridge to thereby impress a signal on the input or grid control of tube l2 which signal will be amplified and applied to winding 40 of the two-phase motor. Excitation of winding 40 by a voltage out of phase with -the voltage across winding 42 "will cause rotation of the motor. Rotation of the motor will change the value of resistance 12 acting in the bridge circuit tending to bring the bridge circuit back into balance. The direction of rotation of the motor is determined by the direction of unbalance in the bridge. After the value of resistance 12 has been changed, due to rotation of arm '15, to where the bridge is once more in balance there will be no signal input impressed on the grid of tube I2. With no input signal, that is, with a balanced bridge, the amplifier output will be zero and the motor will come to rest.

It is readily seen that should the bridge become unbalanced by a definite temperature deviation from the predetermined setting the motor will run until the balance has been re established, by rotation of rheostat 12. Therefore, within the design limits of rheostat 12, the system will manifestly produce an output proportional to the temperature deviation to thereby move the control arm to a predetermined position. However, should the temperature deviation exceed the design limits of rheostat 72 so that arm 44 is swung into engagement with either of stops 56, slip clutch 54 allows for con-- ing current source, an electrical motor having a winding connected to said source, a control arm having limited rotation and drivably connected to said motor, a slip clutch arrangement interposed between the motor and control arm to permit the continued rotation of said motor at times when said control arm has reached its rotational limits, an amplifier input circuit including a bridge having a, plurality of legs, a second winding in the motor inductively related to said first mentioned winding and having connections to said bridge, a third winding in said motor arranged to be excited in response to a signal from said amplifier input circuit, a phase shifter in said connections to said bridge for causing the current flowing in the third winding in response to said signal to be out of phase with the current in the first mentioned winding, said phase shifter being disposed in the system to operate at an energy level lower than the energy level of the motor rating. an amplifier output circuit connected to said second winding for carrying current the fiow of which is influenced by said signal, an element in one leg responsive to temperature changes for creating an unbalanced condition in said-bridge whereby a signal is impressed in the amplifier input circuit, and means responsive to rotation of the arm for creating a balanced condition in said bridge.

2. An electrical system comprising a, motor having a plurality of windings, a control arm having limited rotation drivably connected to said motor, a slip clutch arrangement interposed between the motor and the control arm to provide for continued rotation of said motor after said control arm has reached its rotational limits, an amplifier circuit having an output connected to one of said motor windings for exciting the same, an input circuit to said amplifier circuit and including a bridge with a plurality of resistance legs normally balanced but arranged to produce an input signal which influences the excitation of said one winding when unbalanced, an alternating current source connected to a second winding of said plurality oi windings, a third Winding of said plurality of windings inductively related to said second winding and having connections to said bridge, a condenser in said connections for causing a phase displacement in said one winding with respect to the second winding, said condenser being disposed in the connections to operate at an energy level lower than the energy level required to operate the motor, an element in one leg of the bridge responsive to temperature deviations from a predetermined temperature setting for unbalancing said bridge, and a rheostat in another leg of said bridge responsive to rotation of said motor for recontrol arm is made proportional to the temperature deviation from said predetermined temperafile of this patent:

UNITED STATES PATENTS Number Name Date 943,503 .-Wiard Dec. 14, 1909 1,989,829 Specht Feb. 5, 1935 2,144,668 Stoessel Jan. 24, 1939 2,191,997 Side Feb. 27, 1940 2,209,369 Wills July 30, 1940 2,326,853 Harrison Aug. 17, 1943 2,389,939 Sparrow Nov. 27, 1945 ture setting.

- FRANKLIN 0. WISMAN.

REFERENCES CITED The following references are of record in the 

